Natural density (field density) is the mass of a rock sample per unit volume, measured at its natural in-situ moisture content without any drying. It differs from: dry density ($\rho_d$) = mass of solids / total volume (after oven drying); bulk density = total mass / total volume (at any moisture state, not necessarily natural); saturated density = density when all voids are filled with water. Natural density is measured by core weighing or in-place methods.